Method of manufacturing a wind turbine blade root

ABSTRACT

A wind turbine blade root having fully bonded insert bushings with internal threads for mounting bolts for releasable attachment to a hub of a wind turbine is manufactured by arrangement of a plurality of the insert bushings on a first layer of fibre mat arranged on a holder having a plurality of spaced recesses for accommodation of the insert bushings such that the bushings extend largely in a longitudinal direction of the blade accommodated in the spaced recesses. Subsequently, a number of additional layers of fibre mat are arranged on top of the bushings, and the fibre mat layers are consolidated.

The present invention relates to method of manufacturing a blade rootfor a wind turbine blade having fully bonded insert bushings withinternal thread for mounting bolts for releasable attachment to a hub ofa wind turbine, the method comprising the steps of providing a firstlayer of fibre mat, arranging bushings on the first layer of fibre matto extending largely in the longitudinal direction of the blade,providing additional layers of fiber mat on top of the bushings, andconsolidating the fiber mat.

Wind turbines have been used for decades to exploit the energy of thewind e.g. to produce electricity. To reduce the price of electricityproduced by such wind turbines, the size of the wind turbines haveincreased to a current average nominal power of commercial wind turbinesof approximately 1.5 MW, while wind turbines of up to 3 MW are underdevelopment, and it is expected that even larger wind turbines will bemarketed in the coming years. Common commercial wind turbines have threeblades, which by a 1.5 MW wind turbine have a length of approximately 35m.

The blades are subject to large forces and bending moments inter aliadue to the wind pressure and due to the weight and rotation of theblades, and further the blades are subject to fatigue because of thecyclic load. For example, during one revolution, the blade travelsthrough a region of maximum wind load in the upper part of the circle,whereas the blade experiences a low wind area (or even lee), when theblade passes the tower, and further the wind is normally not constant,as there may be gusts of wind. Naturally the root of the blade and theconnection of the blade to the hub must be able to withstand the load ofthe blade, and a failure of the blade root or the hub would bedevastating and potentially fatal to persons near the wind turbine.

Over the years different approaches have been tried out, as can be seenin U.S. Pat. No. 4,915,590 that discloses a wind turbine bladeattachment method. This prior art blade attachment comprises fibre glasssucker rods secured in the blade root, which sucker rods are unbonded tothe blade root for a substantial portion forming a free end at the rootend, and further the free end of the sucker rods are recessed from theblade root end, which means that the sucker rods can be put undertension. The patent indicates that the sucker rods may be unbonded tothe rotor blade for approximately 85% of the length. The sucker rods aretapered down in diameter toward the secured end in the bonded area,where the rod is mated internally to the blade. Although this may beappropriate for relatively small blades used on wind turbines in August1987, when this US-application was filed, this prior art construction isnot suited, however, for the relatively large blades currently used, asthe sucker rods will not be able to withstand the very large forcespresent at the blade root of large blades, especially as the rods areonly bonded to the blade root to a very limited extent.

In the blade attachment of WO-A2-01/42647, the blade is connected to thehub by bolts screwed into inserts provided in radial holes in the bladeroot. It is a disadvantage however, that radial holes must be providedin the blade root, as these holes seriously weakens the construction andprovides a stress concentration, which means that the blade root must beconstructed to be very strong and hence heavy, which again stresses theconstruction.

A similar construction is described in U.S. Pat. No. 6,371,730, whichdiscloses a blade connected to the hub by bolts screwed into nutsinserted into radial blind holes in the blade root. Although the holesare not through-going, they nonetheless seriously weaken the blade root,and hence this construction is also not advantageous.

It has also been tried to provide a blade root with fully bonded orembedded bushings each having a projecting threaded bolt part, asdisclosed in U.S. Pat. No. 4,420,354. This prior art incorporatesdrilling a relatively large axially extending hole in the blade rootmade of a wood-resin composite, in which hole the bushing, having apreformed resin sleeve, is resin bonded. With this prior art arelatively large amount of blade root material is removed, which weakensthe construction, so the blade root must be overdimensioned. Especiallywith large blades of modem composites like fibre-reinforced plastics,which are relatively flexible, stress concentration at the end of thebushings may be detrimental, as the bushings are significantly morestiff. Moreover this prior art method is somewhat destructive, and asfibre composites for the blade root are quite expensive, andincreasingly will be as larger blades are developed, as it is expectedthat high-tech materials like carbon fibre composites will beintroduced, this procedure is not favorable.

In general, prior art methods of the kind set forth are quite laborintensive and time consuming, as the bushings are spaced by blocks ofe.g. a foam material, and the blocks and the bushings must be arrangedcarefully. Further there is a risk of air pockets being formed in theblade root between the bushings and the blocks, and such air pockets,which are difficult to detect, will seriously deteriorate the strengthof the blade root.

It is an object of the present invention to provide a method of the kindset forth to enable production of a lightweight wind turbine bladehaving an 30 attachment of high strength.

To achieve this object the method according to the invention ischaracterized by an initial step of providing a holder having spacedrecesses for accommodating the bushings, arranging the first layer offibre mat on the holder and arranging the bushings in said recesses.

According to an embodiment, the method comprises the additional the stepof compacting the fibre mats using vacuum mats, whereby a firmcompacting is achieved and the risk of pockets of gas being entrapped inthe composites is significantly reduced.

The mats may be dry mats, only containing reinforcing fibres. Accordingto an embodiment, however, the mats are of a pre-preg type, whereby theblade may be produced in a very efficient way, as the whole blade may beconsolidated in one piece after laying up of the composite, e.g. byheating to cure a thermosetting binder included in the composite.

In a preferred embodiment of the method use is made of insert bushingscomprising a first portion and an extension portion having graduallyincreased flexibility in the direction away from the first portion. Thefirst portion may have any desirable shape, according to an embodiment,however, the first portion of the bushing is substantially cylindrical.Hereby relatively simple and hence cost effective bushings can beachieved, and further a bushing having a substantially cylindrical firstportion will take up relatively little space in the composite materialof the blade root.

Hereby is achieved that the bushings, which are embedded in the bladeroot, and hence are integral therewith, at the same time may provide astrong threaded connection with a bolt for attachment to the hub of thewind turbine, and provide a relatively flexible tip, thereby avoidingdevelopment of stress-concentrations thereby a very lightweight bladehaving an attachment of high strength is hence achieved.

Whereas the first portion of the bushing may have any desirable shape,it is preferred, however, according to an embodiment that the firstportion of the bushing is substantially cylindrical. Thereby relativelysimple and hence cost effective bushings can be achieved, and further abushing having a substantially cylindrical first portion will take uprelatively little space in the composite material of the blade root.

Preferably, the bushings are metallic, although non-metallic bushingse.g. made of high-strength polymers or polymer composites, can beprovided. The extension portion of the bushing may be chamfered toprovide a gradually reduced cross section, whereby a gradually increasedflexibility is achieved in a very simple way. Further by reducing thecross-section of the extension portion of the bushing, a smoothtransition between the first portion of the bushing and the compositeconstruction of the blade root in the direction towards the blade tip.Moreover the risk of pockets of air or gas being trapped in theconstruction at the bushings is greatly reduced by this smoothtransition.

The internal thread of the bushings may extend over the entire length,it is preferred, however, that a first portion of the first portion isthreadfree. Thereby a bolt introduced into the bushing and engaged inthe thread may be put under tension, such that the blade root can bekept engaged with the hub at all times during the cycle of the blade,independent of the cyclic load on the blade, which load comprisestension and compression forces, and bending and torsion moments.

Although it may be advantageous in some occasions to provide thebushings with barbs, radial flanges or the like on the external surfacesthereof, it is preferred according to an embodiment to use bushingshaving smooth external surface.

To provide extra grip with the binder for bonding the bushings, theexternal surfaces of the bushings may be slightly roughened, such as byetching, sand blasting or the like.

In the following the invention will be described in more detail by wayof example and with reference to the schematic drawing, in which:

FIG. 1 is a plan view of a wind turbine blade,

FIG. 2 is an end view of a blade root,

FIG. 3 is a longitudinal section of a bushing,

FIG. 4 is an end view of a blade during manufacture,

FIG. 5 is a sectional view of the blade root,

FIG. 6 is a longitudinal section of the blade root along line VI-VI inFIG. 5, and

FIG. 7 is a longitudinal section of the blade root along line VII-VII inFIG. 5.

A wind turbine blade 1 can be seen in plan view in FIG. 1. The blade 1comprises an aerodynamically designed portion 2, which is shaped foroptimum exploitation of the wind energy, and a blade root 3 forconnection to a hub (not shown) of a wind turbine 20. The blade root 3is a heavily strained area of the wind turbine blade, as the windturbine blade is subject to large forces due to the wind, the rotatingmasses etc. As an example common wind turbine blades 1 for a 1.5 MW windturbine measures approximately 35 25 m and the mass of each blade isapproximately 6,000 kg. Most modem wind turbine blades comprise acentral, hollow high strength beam and an aerodynamic covering havingonly limited strength, and normally both the beam and the covering ismade from a composite material of e.g. fibre reinforced plastics. Otherblade designs comprise an I-beam or no beam in that the covering isreinforced to be self-supporting.

The blade root 3 can be seen in more detail in FIG. 2, which is an endview of the blade root 3. For releasable connection to the hub of a windturbine, the blade root 3 comprises a plurality of bushings 4, of whichonly a few is shown, embedded in the blade root 3, so that bolts (notshown) can be screwed into an internal thread of the bushings 4 for firmbut releasable engagement therewith.

FIG. 3 is a longitudinal section of a bushing 4, which comprises a firstportion 6 and an extension portion 7 having gradually reducedcross-section to a pointed or nearly pointed end 9, so the extensionportion has a gradually increased flexibility. The flexibility could ofcourse be provided by other means, such as providing slits or othercut-outs in radial or axial direction, as will be evident to the skilledperson. The two portions 6, 7 may be integral or provided as individualparts, which may be connected permanently or releasably, such as bythreading, welding, soldering, press-fitting etc. It is presentlypreferred to produce bushing 4 of two independent parts and releasablyjoin these by a threading, although a more permanent connection couldalso be used, such as by gluing, welding, brazing or the like. Byproducing the bushing 4 of two separate parts, machining of theextension portion 7 is more easily performed, and the thread 5 is moreeasily machined in the bore of the bushing. A M30 thread was used in anembodiment for a 35 m blade. As an alternative, the thread 5 may beprovided in the extension portion 7, and the extension portion 7 may bepressfit into the first portion 6.

As schematically illustrated in FIG. 3, the bushing 4 comprises aninternal thread 5. By providing a thread-free proximal portion 11extending from the blade root end 10 of the bushing 4, a bolt screwedinto the thread 5 can be put under tension and hence act as a tie rod.Hereby a firm connection with the hub can be achieved, and the bolt willbe under tension in operation of the wind turbine. Alternatively thebushing 4 may comprise an internal thread 5 along the entire lengththereof, whereas the bolt may be provided with a thread near the tipthereof only, whereby the same result is achieved in that the bolt willbe under tension.

FIG. 4 schematically illustrates a first step in the manufacture of theblade root, which is illustrated in more detail in FIGS. 5-7. The bladeroot 3 is part of the beam, which preferably is made of two parts, whichare assembled after hardening. The blade root 3 is substantiallycircular, and hence made up of two parts of semicircular cross-section.At least one layer of fibre mat is placed in a mould (not shown), and aholder 12, e.g. made of a foam material, is placed on the fibre mat. Theholder 12 has a number of spaced recesses 13 for accommodation of thebushings 4. Prior to placing the bushings 4 in the recesses 13, theholder 12 is lined on the inner side covering the recesses 13 with atleast one first layer 14 of fibre mat, and the outer side of the holder12 is provided with an adhesive 19 and a shell laminate 20. The bushings4 are then placed in the recesses 13 of the holder 12 on top of thefibre mat layer 14 and fixed to a root plate (not shown) for correctpositioning thereof. Fibre glass strips 16, constituting the so-calledmid plane, are arranged between the bushings 4 to extend in thelongitudinal direction of the blade. Four to five layers of fibre glassstrips 16 arranged on top of each other was used in an embodiment. Ascan be seen in FIG. 7, the layers of fibre glass strips 16 extended atleast along the full length of the bushings 4, and were of differentlength, so a smooth transition area was achieved.

In the embodiment according to FIG. 6, the bushing 4 is made up of twoseparate parts, namely the first portion 6 and the extension portion 7.The first portion 6 comprises an internal thread at the end forengagement with an external thread of the extension portion 7. Bothportions 6, 7 are provide as hollow pipes, however the extension portionis chamfered or ground to provide an extension member having graduallyincreased flexibility in the direction away from the blade root 3towards the tip of the wind turbine blade. As can be seen in FIG. 6, astopper 17 is arranged adjacent the thread 5 in the open end of thebushing 4, to avoid entry of epoxy etc. to the thread 5. Further a foamwedge 18 is arranged in the recess of the bushing 4, to ensure a securebonding of the bushing and avoid air pockets in the laminate. Additionalfibre mat layers 15 are arranged on the bushings 4, so a blade root oflaminated construction is provided. When the composite constructionblade root is finished, the construction is preferably compacted, e.g.by means of vacuum mats. Hereafter the mats are hardened, such as byapplying a binder, such 30 as epoxy by spraying or the like. Thebushings 4 are bonded in the laminated blade root construction along thefull length of the bushing 4 from the blade root end thereof to thepointed or nearly pointed end 9 thereof, to provide a secure anchoringof the bushing 4 in the blade root 3. After hardening, the root plate isdetached from the bushings 4. Preferably the mats are thermosetting, andin this event, the mould accommodating the blade root is heated to acuring temperature. Suitable mats include fibre mats of so-called SPRINTand pre-preg materials, as supplied by the Gurit Heberlein Group,Switzerland, comprising a resin, such as epoxy. The temperature forthermosetting these materials is approximately 120° C. By mats should beunderstood any kind of web, fabric, mesh etc. made by e.g. weaving,braiding, knitting or some kind of intermingling of filaments ofreinforcing fibres, and optional filaments of thermoplastic fibres oranother type of binder. The mats should preferably extend in thelongitudinal direction of the blade to provide a smooth transitionbetween the blade root and the beam.

It is found that the blade according to the invention weighsapproximately 4,500 kg, whereas prior art blades weigh approximately6,000 kg, i.e. a reduction of 25%. Clearly this is a large reduction,which will make handling of the blade during manufacture, transport andfixation thereof much easier and less costly. Further lighter bladesmeans reduced load on the structural parts of the wind turbine.

As an example 54 bushings were embedded in the root of a blade for a 1.5MW turbine having three blades measuring 35 m and each bushing had atotal length of approximately 80 cm. Of course the number and dimensionsof the bushings depend on parameters such as material (strength,flexibility etc.) and the shape of the bushings.

1. A method of manufacturing a blade root of wind turbine blade, havinga plurality of fully bonded insert bushings having a longitudinal holewith internal thread for mounting bolts for releasable attachment to ahub of a wind turbine, the method comprising the steps of: providing ana segmented annular holder having a plurality of spaced recessesprovided in an inner circumferential surface of the holder; lining theinner circumferential surface of each segment of the holder with a firstfibre mat layer such that said first fibre mat layer is at leastpartially received in the recesses; respectively placing the pluralityof insert bushings in said recesses such that said bushings contact thefirst fibre mat layer and extend substantially in the longitudinaldirection of the blade; providing an additional fibre mat layer over theinsert bushings such that the insert bushing are substantially encasedbetween the first fibre mat layer on an outer radial side of the insertbushings and the additional fibre mat layer on an inner radial side ofthe insert bushings; and consolidating the fibre mat layers.
 2. Themethod according to claim 1, further comprising the additional steps ofusing mat layers of a pre-preg type, and heating the blade root toconsolidate the pre-preg mat layers.
 3. The method according to claim 2,wherein said first portion of the insert bushing is substantiallycylindrical.
 4. The method according to claim 2, wherein the insertbushings are metallic.
 5. The method according to claim 2, wherein aproximal portion of said first portion of each insert bushing isthread-free.
 6. The method according to claim 2, wherein externalsurfaces of insert bushings are smooth.
 7. The method according to claim2, wherein external surfaces of insert bushings are slightly roughened.8. The method according to claim 1, further comprising the step of usingeach insert bushing comprising a first portion and an extension portionhaving gradually increased flexibility in the direction away from thefirst portion.
 9. The method according to claim 8, wherein the extensionportion of each insert bushing is chamfered to provide a graduallyreduced cross-section.
 10. The method according to claim 1, furthercomprising the additional step of compacting the fibre mat layers usingvacuum mats.
 11. The method according to claim 1, wherein each segmentof the holder has a semicircular cross-section.
 12. The method accordingto claim 11, further comprising the additional steps of using mat layersof a pre-preg type, and heating the blade root to consolidate thepre-preg mat layers.
 13. The method according to claim 1, wherein saidinsert bushings respectively include substantially cylindrical firstportions.
 14. The method according to claim 1, wherein the insertbushings are metallic.
 15. The method according to claim 13, wherein aproximal portion of said first portion of each insert bushing isthread-free.
 16. The method according to claim 1, wherein externalsurfaces of said insert bushings are smooth.
 17. The method according toclaim 1, wherein external surfaces of said insert bushings are slightlyroughened.
 18. The method according to claim 1, wherein each insertbushing includes a first portion and an extension portion, the extensionportion having gradually increased flexibility in a direction extendingaway from the first portion.
 19. The method according to claim 18,wherein the extension portion of each insert bushing is chamfered toprovide a gradually reduced cross-section.
 20. The method according toclaim 1, further comprising the additional step of compacting the fibremat layers using vacuum mats.
 21. The method according to claim 1,further comprising the step of arranging respective fibre glass stripsbetween adjacent bushings.
 22. The method according to claim 21, whereinthe arranging step comprises arranging multiple fibre glass stripsbetween adjacent bushings.